Production of protective layers on cobalt-based alloys

ABSTRACT

COBALT-BASED ALLOYS HAVE THEIR SURFACE TREATED TO PROVIDE HIGH-TEMPERATURE CORROSION RESISTANCE, BY DIFFUSING NICKEL, A METAL OF THE PLATINUM GTOUP, AND ALUMINUM INTO THE SURFACE OF THE ALLOY. THE METALS MAY BE HEAT-DIFFUSED SIMULTANEOUSLY OR IN SUCCESSION.

SP 19, 1972 K. BUNGARDT ETAL PRODUCTION OF PROTECTIVE LAYERS ON COBALT-BASED ALLOYS Filed Dec. 5, 1970 United States IPatent Office 3,692 554 PRODUCTION F PROTECTIVE LAYERS ON COBALT-BASED ALLOYS Karl Buugardt, Gunter Lehnert, and Helmut Meinhardt,

Krefeld, Germany, assignors to Deutsche Edelstahll ABSTRACT OF THE- DISCLOSURE Cobalt-based alloys have their surface treated to provide high-temperature corrosion resistance, by diffusing nickel, a metal of the Platinum Group, and aluminum into the surface of the alloy. The metals may be heat-diffused simultaneously or in succession.

This invention relates to a method of producing on the surface of parts made of cobalt-based alloys protective, diffusion layers that are resistant at high temperatures to oxidation, scaling and corrosion.

It is a known practice to protect objects, for example turbine blades for jet propulsion engines which usually consist of high temperature metal alloys, from oxidation, scaling and corrosion, by diffusing aluminum into their surfaces thereby to provide a protective layer thereon.

The object of the present invention is to provide on parts made of high temperature metals, particularly cobalt-based alloys, a protective layer which imparts irnproved resistance to oxidation, scaling and corrosion at high temperatures.

For achieving this object the present invention consists in applying to the parts, possibly after they have been previously cleaned and degreased, nickel, one or more metals of the platinum group of metals of the `Periodic System of Elements (preferably platinum), and aluminum, either simultaneously or in succession, and to diffuse the said metals into the surface of the parts.

In a useful embodiment of the method of the invention, layers of nickel, of metal(s) of the platinum group and of aluminum may be consecutively applied to the surface of the part, for example by mechanical or electrolytic means, dipping, spraying or otherwise, and the metals in the layers thus formed diffused severally or jointly into the surface of the part by a diffusion heat treatment.

In an alternative method of carrying out the invention a layer of nickel and then a layer of one or more metals of the platinum group may be electrolytically deposited and the aluminum then deposited from a gas phase on the surface of the part and diffused into the surface of the part together with the nickel and the metal(s) of the platinum group. With advantage the part that has first been provided with a nickel layer and then with a layer of metal(s) of the platinum group, may be embedded in a powder mixture containing metallic aluminum possibly associated with a small proportion of a metal halide and an inert filler (e.g. alumina), the aluminum being then deposited on the part from a gas phase and finally diffused into the surface of the part together with the nickel and the metal(s) of the platinum group. Such a mixture may for example comprise about of aluminum and about 95% of the inert filler.

Cobalt-based alloys that have been provided with a protective diffusion layer according to the invention last about twice as long as similar parts that have been provided with a protective aluminum diffusion layer without nickel and platinum. The graph of the accompanying 4 Claims 3,692,554 Patentedv Sept. 19, 1972 drawing having time, expressed in hours', on the abscissa and change in weight, expressed in milligrams per square centimeter, on the ordinate shows, as a function of operating time, the change in weight-of test pieces that have been aluminized by a conventional method, and that have been protected by the method according to the invention. It will be seen that the reduction in weight which indicates the rate at which material is lost from the surface of the tested part by oxidation, scaling and corrosion, is less for p'arts protected according to the invention` than parts that have been merely aluminized, and that their useful life of 600 to 700 hours in scaling test when exposed to burnt natural gas at 1100 C. with an air factor of 1.2, is twice as long. The material of all the tested parts was a cobalt alloy containing I0.45% carbon, less than 0.5% silicon, less than 0.5% manganese, r21.0% chromium, less than 1.0% nickel, 11.0% tungsten, 2.0% nio# bium or tantalum, 1.75% iron, balance cobalt. The protective treatment of the parts accordingv to the invention was as follows: .4 v

PRELIMINARY SURFACE TREATMENT Electrolytic composition of bath:

240 g./litre of nickel chloride (NiClZ-HZO) 120 g./litre of 36% hydrochloric acid. Conditions of electrolysis: temperature 45 C. and current density 10-11 amps/sq. dm. Thickness of nickel deposit: 20 microns.

Heat treatment 2 hours at 260 C. followed by 3 hours at 450 C.

PRELIMINARY SURFACE TREATMENT OF THE NICKEL BEFORE PLATINISATION Degreasing in a cyanide degreasing bath as hereinbefore described cathodically 10 seconds, anodically 2O seconds and cathodically l0 seconds at a temperature of 20 C. and a current density of 5 amps/ sq. dm., followed by pickling in 10% by weight caustic soda cathodically 3 seconds and anodically 10 seconds at a temperature of 50 C. and a current density of 10 amps/sq. dm., followed by pickling in 4% hydrochloric acid cathodically 2 seconds and anodically 30 seconds at a temperature of 40 C. and a current density of 2.5 amps/sq. dm., followed by washing in warm water at C.

Platinum deposition Electrolytic composition of bath:

13 g./litre of hexachloroplatinic acid, HzPtCl 45 g./litre of triammonium phosphate, (NH4)3PO4 240 g/litre of disodium phosphate, Na2HPO4.

The result of this treatment was an overall thickness of the protective diffusion layer of 60 microns.

Similar results are obtainable with palladium, rhodium and/or ruthenium when used instead of platinum.

What is claimed is:

l. A method of producing protective di'usion layers that are resistant to oxidation, scaling and-corrosion at high temperatures on the surface of a cobalt-based alloy part, comprising the steps of depositing successively first nickel,r and then one or more metals of the group consisting of platinum, rhodium, ruthenium and palladium onto the surface of the said metal part, heating said coated part to, diiusion temperature while at the same time` aluminum from a gas phase is diffused into the surfaceA of the said coated metal part.

A method according to claim 1 in 4which the said metal part4 coated with nickel and then with a metal layer of one or more metals of the group consisting of platinum, rhodium, ruthenium and palladium is embedded in a powder mixture comprising metallic aluminum and anfinert filler, and subjected in said powder mixture to a, diffusion treatment whereby aluminum from the gas phase diffuses ,into the surface of the said metal part.

3. A method according to claim 2 wherein the said powder mixture comprises about 5% by weight of metallic aluminum and about 95% by weight of inert filler.

4. A method according Ainert filler is alumina.

References Cited UNITED STATES PATENTS 3,107,175Y 1,0/ 1963 Cape v .v `1,17--22 X 3,079,276 v2/1963 vVl'uyear et al 117-131 X 3,494,748 2/1970 Todd, 117f131 X 2,664,873 1/ 1954 Graham V v -a 1174-22 X 2,682,101 6 /19544 Whitfield et al. 117-71 MX 2,917,818 Y121'1959 Thomson j f 7204-7-38 SX FOREIGN PATENTS 1,553,233 1/1969 France 11T- 107.2 P

ALFRED L. LEAVIT T, Primary Examiner I. R. BATTEN,"IR., Assistant Examiner-vv U.S. Cl. X.R. 

